Various non-invasive techniques have been developed for determining blood-related parameters such as hemoglobin, hematocrit, oxygen saturation, etc. These techniques are disclosed for example in the following publications:
A pulse oxymetry based hemoglobin measurement technique is described in the article “Noninvasive total hemoglobin measurement”, by Kye Jin Jeon et al., Journal of Biomedical Optics 7(1), 45-50, January 2002. This technique consists of a wavelength selection and prediction algorithm for determining total hemoglobin concentration. A model has been developed, based on the difference in optical density induced by the pulsation of the heart beat, by taking an approximation of Twersky's theory on the assumption that the variation of blood vessel size is small during arterial pulsing. The device utilizes a five wavelength light emitting diode array as the light source. The selected wavelengths are two isobestic points and three in compensation for tissue scattering. Data are collected from 129 outpatients who are randomly grouped as calibration and prediction sets. The ratio of the variations of optical density between systole and diastole at two different wavelengths is used as a variable. Several such variables have been selected that show high reproducibility among all variables. Multiple linear regression analysis has been made in order to predict total hemoglobin concentration. The correlation coefficient is 0.804 and the standard deviation is 0.864 g/dL for the calibration set. The relative percent error and standard deviation of the prediction set are 8.5% and 1.142 g/dL, respectively. These investigations demonstrate the possibility of noninvasive hemoglobin measurement, particularly, using the wavelengths below 1000 nm.
U.S. Pat. No. 5,277,181 discloses noninvasive measurement of hematocrit and hemoglobin content by differential optical analysis. This technique utilizes differential optical absorption of two or more wavelengths of light during blood volume changes. The method is also useful for noninvasive measurements of other blood analytes, such as glucose, where variations in hematocrit or blood hemoglobin concentration cause errors in the measurement.
U.S. Pat. No. 4,927,264 discloses a non-invasive measuring method and apparatus of blood constituents. Here, in order to measure the oxygen saturation in venous blood, a venous blood stream is made time-variant by applying pressure with a peak value of the minimum blood pressure to a proximal portion from a measuring part. Light beams with different wavelengths are transmitted from the measuring part and detected by photodiodes. Photodetected signals are logarithm-converted and venous signal components are separated from logarithm-converted signals with a filter circuit. The oxygen saturation of venous blood is calculated on the basis of separated venous signal components.
U.S. Patent No. 5,827,181 describes a noninvasive blood chemistry measurement method and system that isolate measurement contributions due to a patient's blood to accurately measure blood chemistry. According to one embodiment, a noninvasive blood chemistry measurement method decreases the blood volume within a patient's body part relative to the normal blood volume in the body part and performs a baseline measurement. Blood volume is then increased and a second measurement is performed. Comparison of the second measurement to the baseline measurement isolates the measurement attributes of the patient's blood. In accordance with another embodiment, a noninvasive blood chemistry measurement system decreases blood volume by applying mechanical pressure to a body part. According to yet another embodiment, blood volume in the body part is decreased using a pressure cuff. In a further embodiment, a noninvasive probe accurately measures blood chemistry and uses a suction cup to increase blood volume at the blood chemistry measurement site.
U.S. Pat. No. 6,606,509 discloses a method and apparatus for improving the accuracy of noninvasive hematocrit measurements. According to this technique, the changes in the intensities of light of multiple wavelengths transmitted through or reflected light from the tissue location are recorded immediately before and after occluding the flow of venous blood from the tissue location with an occlusion device positioned near the tissue location. As the venous return stops and the incoming arterial blood expands the blood vessels, the light intensities measured within a particular band of near-infrared wavelengths decrease in proportion to the volume of hemoglobin in the tissue location; those intensities measured within a separate band of wavelengths in which water absorbs respond to the difference between the water fractions within the blood and the displaced tissue volume. A mathematical algorithm applied to the time-varying intensities yields a quantitative estimate of the absolute concentration of hemoglobin in the blood. To compensate for the effect of the unknown fraction of water in the extravascular tissue on the hematocrit measurement, the tissue water fraction is determined before the occlusion cycle begins by measuring the diffuse transmittance or reflectance spectra of the tissue at selected wavelengths.
A different approach is disclosed in various patents assigned to the assignee of the present application, such as for example U.S. Pat. No. 6,400,972; U.S. Pat. No. 6,587,704; U.S. Pat. No. 6,711,424; and U.S. Pat. No. 6,804,002. These techniques provide for measurement of various parameters of the patient's blood, based on the creation of a condition of artificial kinetics at a measurement location, and maintaining this condition during a certain time. Measurements are carried out during a time period including this certain time by applying an external electromagnetic field to the measurement location and detecting a response to the applied field. Measured data is in the form of time evolutions of the responses of the medium corresponding to the different parameters of the applied field. By analyzing the measured data, at least one blood parameter is extracted.